The present invention relates to a heat curable organopolysiloxane composition or, more particularly, to a heat curable organopolysiloxane composition capable of giving a cured product having no surface tackiness and a high hardness and being excellent in anti-crack resistance, heat resistance and electric properties.
As is well known, there have been known several types of heat curable organopolysiloxane compositions. For example, heat curable organopolysiloxane compositions comprising an organopolysiloxane having, in a molecule, vinyl groups, phenyl groups and methyl groups directly bonded to the silicon atoms and an organic peroxide are disclosed in Japanese Patent Publications 31-9394, 34-1288 and 36-11444.
One of the defective points in the heat curable compositions of the above described type is the surface tackiness of the cured product when the composition is cured by heating in air due to the inhibiting influence of the atmospheric oxygen on the curing in the surface layer causing various troubles in the practical use of the composition. Therefore, the use of such a curable composition is limited only to the applications where curing can be carried out with exclusion of atmosphere air unless certain special curing means are undertaken or the surface having tackiness is subjected to a secondary treatment to remove the tackiness.
In order to overcome the above described disadvantage, there has been proposed a method in which the composition is admixed with an organic diluent having unsaturated linkages to improve the curing of the composition in the surface layer. See, for example, Japanese Patent Publication 53-11320. Use of such an organic diluent is not always recommendable because the otherwise excellent heat resistance of the cured silicone products is greatly decreased by the addition thereof along with other disadvantages in the coloration and lowering of electric properties, in particular, at an elevated temperature.
On the other hand, a heat curable organopolysiloxane composition of another type utilizes the crosslink formation by the addition reaction between vinyl groups of a vinyl-containing organopolysiloxane and hydrogen atoms directly bonded to the silicon atoms in an organohydrogenpolysiloxane taking place in the presence of a platinum catalyst to give a cured product. Curable compositions of this type are described, for example, in Japanese Patent Publications 34-990, 38-26771, 48-34382, 52-39658, 52-44900 and 53-21027.
Curable compositions of this type, however, can hardly give a cured product having high mechanical strengths and electrical properties, in particular, at an elevated temperature. This difficulty is partly overcome when large part of the organopolysiloxane constituent is a resinous organopolysiloxane composed mainly of trifunctional siloxane units in its molecular structure. Such a composition usually has a high viscosity so that the workability is poor unless it is used as diluted with an organic solvent. For example, when such a composition is used for impregnation or molding, troubles are sometimes made in the formation of voids or entrainment of air due to the poor fluidity of the composition. Decreasing of the trifunctional siloxane units to obtain lower viscosity and, in compensation therefor, increasing the content of functional groups to increase the crosslinking density and to obtain a cured product having the same degree of hardness as those obtained from the above described compositions necessarily lead to an inferior anti-crack resistance of the cured product which is readily cracked by a mechanical shock or a thermal shock.
Among the problems in the above described heat curable organopolysiloxane composition of the addition reaction type between vinyl groups and silicon-bonded hydrogen atoms in the presence of a platinum catalyst, the largest is the balance between the viscosity of the composition and the mechanical strengths or anti-crack resistance of the cured products thereof. The attempt hitherto undertaken for decreasing the viscosity of the composition and improving the properties of the cured products is to admix a low molecular weight cross-linking agent or a polymerizable diluent with the composition. This attempt has been not always successful because of the complexness of the manufacturing process and the expensiveness of the adjuvant materials leading to an increased production cost as well as the inferior electric properties at an elevated temperature or increased volatile matter after heating of the cured products due to the addition of the low molecular compounds.